The invention relates to an apparatus for treating water by means of an electrical field comprising a treatment chamber and an anode and a cathode mounted therein. When a preferably inhomogeneous electric field is applied between the anode and the cathode, seed crystals of minerals contained in the water are formed on the cathode. These seed crystals are separated from the cathode by separating or stripping means and carried away with the flowing water.
Drinking water contains essential minerals, among others calcium and magnesium carbonates. Such minerals, in total, are called water hardness. On one hand, such minerals are important under health aspects. On the other hand, they may cause malfunction of the drinking water installation, in particular if the water is heated. When the water is heated in a drinking water heater, for example, insoluble lime depositions, also called boiler scale, are formed. This boiler scale is formed on the warm side of the installation. Boiler scale impedes heat transmission. In tubes, the boiler scale, over the years, may result in closure of the tube installation downstream of the drinking water heater.
In order to avoid such depositions of boiler scale, the drinking water has been xe2x80x9csoftenedxe2x80x9d. This softening process removes the minerals from the drinking water. Such softening can be achieved by ion exchange or reverse osmosis. This avoids the formation of boiler scale. In exchange therefor, however, the quality of the drinking water is reduced by the removal of the minerals, which, for other reasons, are desirable.
Methods of treating drinking water by means of a so-called xe2x80x9celectro-physical apparatus are known. Such electro-physical apparatus are to cause the formation of seed crystals in the drinking water. The seed crystals are carried away by the drinking water. The conglomeration of the seed crystals results in the drinking water thus treated causing less deposition in the form of boiler scale, on one hand. On the other hand the minerals in the drinking water are retained.
The treatment by means of electro-physical apparatus is substantially based on the following principle:
Drinking water always contains a certain proportion of carbon dioxide (CO2) dissolved therein. This carbon dioxide forms a reaction equilibrium with the remaining components, calcium hydrogen carbonate (Ca(HCO3)2 being formed from CaCO3 in accordance with the reaction equation
CaCO3+H2O+CO2Ca(HCO3)2
If the water is heated, CO2 will escape from the water and boiler scale will be formed. It is, however, also possible to change purposefully the lime-carbon dioxide equilibrium in the drinking water.
If carbon dioxide is supplied to the water, this will result in shifting the equilibrium to the left in the above equation. Seed crystals of calcium carbonate (lime over-saturation) are formed from the calcium hydrogen carbonate present in the water. Then the subsequently formed calcium carbonate will be deposited predominantly on the once generated seed crystals, i.e. the seed crystals xe2x80x9cgrowxe2x80x9d.
This reaction is caused electrolytically in a treatment chamber containing a cathode and an anode. The seed crystals are formed on the cathode. The seed crystals have to be added to the drinking water after they have grown sufficiently. Then carbonate is already present in the form of seed crystals in the drinking water, when the drinking water is heated. There is no need of generating seed crystals anew. Accordingly, the carbonate will be deposited predominantly on the seed crystals carried away in the water rather than on the installation elements.
It has been found that development of seed crystals is improved if the electric field between the cathode and the anode is inhomogeneous.
EP patent application 0,751,096 discloses an apparatus for the electro-physical drinking water treatment by means of an electric field wherein an electric field inhomogeneous in space is generated by an a.c. voltage superimposed to a d.c. voltage.
A further prior art apparatus comprises a special steel brush serving as cathode on which seed crystals are formed. These seed crystals are separated from the cathode by means of a stripper. This stripper is a comb which is passed through the special steel brush an causes deformation of the flexible xe2x80x9cbristlesxe2x80x9d.
One problem with the formation of seed crystals in an inhomogeneous electric field is that the dwell time of the water with the seed crystals in the electric field is too short. Though many seed crystals are formed, the crystal growth on the seed crystals is not satisfactory. If the current through the electrodes is increased, in order to improve the crystal growth, this results in increased wear of the electrodes. They have to be replaced in regular intervals. This involves high servicing expenditure.
The problem cannot be solved by connecting a plurality of treatment chambers in series. Quite a few treatment chambers would have to be connected in series. Generating an inhomogeneous electric field requires quite an amount of technological expenditure. Furthermore, saturation will be reached: From a certain number of treatment chambers on, each additional treatment chamber will have no effect on the efficiency of the water treatment.
It is an object of the invention to provide an apparatus for water treatment having increased crystal growth.
To this end, an aftertreatment chamber having at least one cathode and at least one anode is provided. In this aftertreatment chamber, the water is passed through an electric field generated between the cathode and the anode. In this aftertreatment chamber, the water has a longer dwell time than in the treatment chamber.
Crystal growth is improved by such an aftertreatment chamber with increased dwell time of the water. In the aftertreatment chamber, the seed crystals generated in the treatment chamber are exposed to an electric field and, thereby, grow considerably faster. Therefore, the efficiency of a treatment chamber provided with an aftertreatment chamber is higher than the efficiency of a simple treatment chamber.
Preferably, the aftertreatment chamber ensures long dwell time of already existing seed crystals. Against this, the treatment chamber preferably is of a design which ensures the generation of as many seed crystals as possible, the dwell time being of no concern. In accordance with a preferred embodiment of the invention, the cathode has a plurality of pins mounted on a carrier. A perforated disc with holes is provided. The pins of the cathode extend through the holes of the perforated disc. The perforated disc cam be moved over the lengths of the pins. By such an arrangement, a strongly inhomogeneous electric field is generated which ensures the formation of many seed crystals. The seed crystals thus formed on the pins of the cathode are stripped therefrom by the perforated disc. An electric motor may be provided to move the perforated disc.
A plurality of treatment chambers may be provided in parallel and/or in series to permit the apparatus to adapt to water flow rate and efficiency of seed crystal formation.
Preferably, the aftertreatment chamber is arranged around the treatment chamber and communicates with the treatment chamber through apertures in a circumferential partition between the treatment chamber and the aftertreatment chamber. Thereby, the apparatus has small dimensions.
Preferably, the aftertreatment chamber is subdivided by radial partitions into a plurality of partial chambers, each aperture in the circumferential partition opening in a respective one of the partial chambers. The radial partitions may be electrodes for generating the electric field in the aftertreatment chamber. Preferably, the electrodes are graphite rods.
In a preferred embodiment, a plurality of treatment chambers are arranged module-like one on top of the other. Then each treatment chamber communicates with an inlet. The aftertreatment chambers are subdivided in partial chambers such that the partial chambers extend over the whole height of all treatment chambers and, at one end, open into an outlet common to all partial chambers. The direction of flow of the water through the treatment chambers is transverse to the pins of the cathode. This structure offers the advantage that an only small pressure drop occurs, as a plurality of treatment chambers are connected in parallel and are surrounded by a common aftertreatment chamber.
Preferably, the electrodes for generating the electrical field in the aftertreatment chamber are contacted to the outside in the aftertreatment chamber on the side opposite the outlet by means of a spring-urged pin. This offers the advantage that the plumber, when installing or dismantling the apparatus, needs not carry out any electrical contacting but only has to connect the inlet and outlet with the pipe system. The electrical contacts can, together, be connected to the mains through a conventional plug.
When the electrodes have alternately positive and negative polarity, an electric field will be generated in the aftertreatment chamber in a simple way. In order to avoid deposition on the electrodes, the polarity of the electrodes is reversed after selected time intervals.
Trough-shaped cavities provided both in the partition between treatment chamber and aftertreatment chamber and, at corresponding locations in the outer wall of the aftertreatment chamber and extending over the length of the electrodes accommodate the electrodes. This permits easy exchange of the electrodes without having to detach any attachment means. The electrodes are retained in the trough-shaped cavities without dislocation.
An embodiment of the invention is described in greater detail hereinbelow with reference to the accompanying drawings.